FIX older pandas versions require python 2.X

Author: hayd

.travis.yml

@@ -5,9 +5,9 @@ env:
   - PYTHON=2.7 PANDAS=0.15.1
   - PYTHON=3.3 PANDAS=0.14.1
   - PYTHON=3.4 PANDAS=0.15.1
-  - PYTHON=3.4 PANDAS=0.12.0
-  - PYTHON=3.4 PANDAS=0.11.0
-  - PYTHON=3.4 PANDAS=0.10.1
+  - PYTHON=2.7 PANDAS=0.12.0
+  - PYTHON=2.7 PANDAS=0.11.0
+  - PYTHON=2.7 PANDAS=0.10.1
 
 install:
   - sudo apt-get update